Electrocardiographic (ECG) measuring systems generally apply 3 electrodes (to the chest or 10 electrodes (4 limbs and 6 specific points on the chest) to the skin, and, through a differential operational amplifier (OP-AMP), report signal differences between a selected pair of electric contacts or electrodes or between an electrode and a summed reference. The electrical activity thus monitored is generated by a sequence of ion movements in the heart that depolarize (release) and then repolarize (rebuild) an ionic charge distribution across cell membranes, that relates to actuation of contraction of the heart muscle. By convention accepted in the art (with reference to the Figures), a “12 lead” ECG consists of lead pairings I, II, III, avR, avL, avF, v1, v2, v3, v4, v5, and v6, where lead I reports the voltage difference between an electrode on the left arm and another on the right arm; lead II left arm vs. foot; lead III right arm vs. foot; Lead aVR reports right arm vs. combined reference of left arm and foot; aVL left arm vs. right arm and foot; aVF foot vs. left arm and right arm; and the v-leads (v1-v6, v for voltage) represent a series of prescribed positions across the front of the chest vs. the combined reference of left arm, right arm and feet. The American Heart Association and the Cardiac Society of Great Britain defined the standard positions and the wiring for the chest leads v1-v6 in 1938 (Barnes A R, Pardee H E B, White P D. et al. Standardization of precordial leads. Am Heart J 1938; 15:235-239). Emanuel Goldberger added the augmented limb leads aVR, aVL and aVF to Einthoven's three limb leads and the six chest leads in 1942, constituting the 12-lead electrocardiogram that is used today. ECG systems are widely used for diagnosis of rhythm changes, metabolic effects, and heart damage.
The ECG signal is commonly described in terms of a sequence of waves called P wave, QRS complex, and the T-wave (originally described by Willem Einthoven, Einthoven W. Ueber die Form des menschlichen Electrocardiograms. Arch f d Ges Physiol 1895;60:101-123; Nobel prize awarded 1924). The QRS complex may consist of just R wave or RS or qR or qS, where q, if present, is an initial down-going voltage deflection, R, if present, is the first up-going deflection after the p-wave, and S, if present, is a subsequent down-going deflection (if there are further up-going and down-going waves in the QRS, those are labeled R′, S′, then R″, S″, respectively). The P-wave corresponds to electric activation of the small chambers of the heart. The R-wave or QRS complex corresponds to electrical activation of the large chambers of the heart. The T wave corresponds to the staggered end of electric charge redistribution recovery from the electrical activation of the large chambers.
Alternatively, the heart has been modeled for simplicity as a 3D electric dipole represented by orthogonal ECG tracings, and xy, xz and yz loop plots known as vectorcardiograms (VCG's), but VCG's are not relied on and are unpopular clinically for diagnostics or monitoring (E. Frank: The Image Surface of a Homogenous Torso, Am. Heart J. 47:757, 1954). Vectorcardiograms are based on 3 orthogonal voltage loop plots representing an electric dipole that changes length and orientation cyclically. The heart is not that simple, so the model introduces error well described in the literature. The vector model is not as powerful at separating unwanted signals as is the multivariate method of this invention, it does not provide ST segment monitoring, and it requires a more difficult set up to be done properly. The underlying model has an estimated 10% error, because the heart is not simply a 3D electric dipole, different lead positions have distinct local information, and more than 4 leads are needed to reproduce the ECG (G. E. Dower, H. B. Machado, J. A. Osbone: On Deriving the Electrocardiogram from Vectorcardiographic Leads, Clin. Cardiol. 3:87, 1980; L. Edenbrandt, O. Pahlm: Vectorcardiogram Synthesized from a 12-lead ECG: Superiority of the Inverse Dower Matrix, J Electrocardiol 21:361, 1988).
ECG's are used to detect the “R-wave” (the initial up-going component of the QRS). R-wave detection is used to synchronize imaging systems with the position of the beating heart, e.g., for triggering data collection (a strobe-like method to collect data at specific times to effectively freeze the motion of the heart), or for gating the data (to sort collected data in relation to the timing of activation of the heartbeats). There has historically been an assumption that finding the R-wave is the best approach to synchronization of imaging systems, however, the real issue for imaging is to find a time when the heart is filled to a matching volume and position so that data from multiple such times can be combined to form a coherent and consistent image. The intervals between R waves affect this for several cycles. In particular, contractility (the rate and strength of contraction) and the filling time and volumes from preceding cardiac cycles influence the position and timing of the subsequent heart beats.
Following the R-wave and before the T wave there is an early electric recovery period reflected by a voltage called the “ST segment.” In certain lead pairings, the ST segment may be depressed or elevated with respect to the baseline of the ECG signal, and in particular with respect to the extrapolation of the P-R segment. It may become depressed when blood supply to the heart is insufficient for the normal metabolism (ischemia), or elevated when there is new or recent damage to the heart muscle (injury current) or transmural ischemia from spasm of a coronary artery, or vice versa if ischemia or damage is visible from the opposite side of the heart. The electrical signals from the heart are not from a point source, but rather represent a summation of contributions from different tissue segments within the heart. Thus ST segment deviations are produced separately by the anterior wall and posterior wall (as well as lateral wall, inferior wall, etc.) which can cause confusion: an anterior lead ST elevation can represent ST elevation produced by the anterior wall and/or ST depression produced by the posterior wall. Thus there is an application of the present invention for routine ECG assessment outside of MRI (in the absence of contamination signals generated by flow in the great vessels and from magnetic field gradients) by applying signal separation to distinguish anterior wall contribution from posterior wall contribution.
Such ST segment deviation is typically evident only in particular lead pairings, which may or may not include standard leads. For example, infarctions on the posterior or right aspects of the heart may be missed in a 12-lead ECG, and an enlarged or unusually positioned heart may not be adequately assessed with the standard 12-lead system. When such circumstances are suspected, clinical practice calls for additional lead placements, e.g., V7, V8, V9, V4R, and V5R.
Continuous monitoring of the ST segment following a heart attack provides a good predictor of the amount of damage. In particular, the intensity and duration of myocardial ischemia (both reflected by the estimated areas under the ST-trend curve) determine the extent of myocardial damage infarct size and ejection fraction in patients with acute myocardial infarction who receive clot-busting therapy (Karel G. M. Moons PhD, Peter Klootwijk MD PhD, Simon H. Meq MSc, Gerrit-Anne van Es PhD, Taco Baardman MD, Timo Lenderink MD, Marcel van den Brand MD PhD, J. Dik F. Habbema PhD, Diederick E. Grobbee MD PhD, Maarten L. Simoons MD PhD. Continuous ST-Segment Monitoring Associated With Infarct Size and Left Ventricular Function in the GUSTO-I Trial, Am Heart J 138(3):525-532, 1999). Also in association with ischemia or injury, the T-wave may change form or invert.
Prior art solutions to problems encountered during electrocardiography include using light emitting diodes to flag poor electrode contact because electrodes may become detached during data collection. One device uses a microprocessor to trigger an alarm when a drop in impedance below a threshold value is detected, simultaneously activating an automatic search for alternative lead combinations that may be intact. Another device applies additional leads to use as alternates depending on patient size, embedding the leads in a uniformly weighted pad. Another prior art device enables amateur application of multiple leads in the general region of the heart for computer selection of a lead that appears to have correct position.
Despite the application of multiple leads, these prior instruments and methods merely provide alternates for selection of a preferred electrode set to use in the conventional manner of reporting signal differences between a pair of voltage sources and/or require particular lead placements. They assume that there is a best subset of standard combinations and standard positions to use for gathering a usual ECG signal. In normal healthy subjects, with standard lead placements, that may be true; but diseased patients generally have changes in the heart resulting in changes in the ECG signal from standard lead pairs. In particular, myocardial infarction, or heart attack, typically results in loss of R-wave height.
Triggering and gating are impaired if the R-wave is not the expected tallest narrow spike in the ECG. Taller R-waves may be found if observed from other, non-standard, electrode locations. To address the problem of failed ECG triggering, filters have been applied ECG signal to reduce signal at frequencies not of interest; that can help but does not reliably resolve the problem. In particular, filtering removes signal based on frequency content, but the frequency content of aortic pulsation is substantively similar to the frequency content of ST-T waves, and therefore the aortic pulsation artifact of concern in MRI cannot be corrected by filtering without distorting the T-wave and interfering with ability to monitor ST-T wave changes.
Arand, et al. (U.S. Pat. No. 5,817,027) disclose a method of identifying temporal components of an EKG (not within an MRI), such as R-waves and T-waves by matching those components to “templates”, which requires the components to be in standard form and appearance. The method uses signal averaging, which blurs random fluctuations and helps suppress uncorrelated noise, but which actually reinforces systematic signals such as the aorta signal seen in MRI which is highly synchronous with the EKG (the MRI aortic signal artifact is not random noise or a mere motion artifact.) That technique does not apply any source separation nor extract information from different sensitivities of sensors to sources of different physical locations of origin. To the extent that the appearance of aorta pulsation signal would be modified by her method, the ST-T wave would also be modified; for the bulk of shared frequency content in ST-T wave and aorta pulsation signal her method has no ability to separate the two.
Even with a normal ECG, there are “electrically silent areas” of the heart in which ischemia or injury may occur without the usual evidence of ischemia or injury in the standard lead position ECG, as mentioned above. Patients with enlarged or repositioned hearts may be better evaluated from non-standard lead positions. As a subject breathes in and out there is a “baseline artifact” which may interfere with standard interpretation of the signals, but which may prove useful in reporting the phase of breathing. Also, there is a variation in the interval from one heart beat to the next (“R-R interval”), allowing increased or decreased filling of the chambers, and thus changes in the size of the heart that may impair the goal of ECG triggering or ECG gating. In response to changes in filling, the heart changes its contractility (strength and rate of contraction) for the subsequent cycle(s). Also, incorrect placement of the chest leads v1-v6 can produce false indications of ischemia or infarction.
Magnetic Resonance Imaging (MRI) is an example of an imaging device that uses the height of the R-wave as a trigger to synchronize data collection to the heartbeat activation and effectively freeze the motion of the heart. Early MRI studies took over 20 minutes to build one or more images of the heart as a composite from multiple heartbeats. New MRI systems can acquire images in less than 20 seconds, with some methods completing an image in less than half a second. With such capabilities, it is now possible to follow changes in the heart from beat to beat. For example, one may observe the arrival of a blood-born contrast agent and determine if there are areas of impaired blood delivery. Such methods need, more than ever, a reliable detection of the electrical activation of the large chambers of the heart. Newer MRI systems also have higher magnetic fields than in the past, resulting in greater induction of an electrical signal due to the pulses of blood moving along in the great vessels. That signal generally adds to the normally lower “T-wave.” Consequently, the R-wave is often not the tallest wave. Also, MRI applies controlled magnetic fields to encode the data it collects for imaging. The newer faster imaging methods use improved hardware to change the magnetic field more quickly, inducing higher, narrower, electric signals that commonly obscure the R-wave. Baseline artifact related to the respiratory cycle may be exaggerated.
It remains desirable to perform accurately medical diagnostic testing on the heart, in the presence of disturbing signals, or with imperfect lead placements, such as in settings where time or expertise are limited. Likewise it remains desirable to obtain diagnostic signals when signal character is non-standard due to disease, or when the signal changes after the subject is advanced into an imaging system. Also, it is desirable to extract information about the respiratory cycle.